Field devices are devices, which have an electronics and which are applied in the field, especially in industrial plants. Field devices include especially measuring devices, which metrologically register a physical, measured variable, e.g. a flow, a pressure or a fill level. Field devices are applied e.g. in industrial measuring- and automation technology. There, they are applied for, among other things, registering information, especially measured values and/or process parameters. The information registered by one or more field devices is transmitted, on a schedule, to one or more receivers. The receivers are, as a rule, superordinated units, such as e.g. computers, controllers and/or process control systems, which process the transmitted information, display it and/or apply it for open- and/or closed-loop control of processes, such as e.g. industrial manufacturing- or processing methods. There are, however, also applications, in the case of which information is transmitted from field device to field device.
Besides information transfer from field device to receiver, frequently also there is information transfer from a superordinated unit to a field device. In this way, e.g., data are transmitted for adjusting, parametering and/or configuring a field device, as well as for matching the field device to tasks, e.g. specific measuring tasks, to be executed by it.
The uni- or bidirectional information transfer between a field device and a receiver, e.g. a superordinated unit or another field device, occurs via a transmission line connected to electronics present in the field device.
Electrical signals bearing the information are transmitted via the transmission line. The various signal forms in industrial use for this purpose ranges from wire-transmittable, analog and digital signals to wirelessly transmittable, high-frequency signals.
Among the analog signal forms is e.g. that frequently referred to in the industry as a 4-20 mA signal. In the case of this transmission form, a signal current flows via the transmission line, e.g. a 2-wire line, and is controlled as a function of a variable, e.g. a measured variable, determined by the field device, to values between 4 mA and 20 mA.
Among the digital signal forms are e.g. digital signals transmittable via bus lines, such as those used in bus line systems, such as e.g. HART, Profibus PA, Profibus DP, ModBus or Ethernet, for uni- or bidirectional digital communication.
Moreover, in increasing measure, also high-frequency signals are applied, such as those used, for example, for wireless communication via digital radio- or mobile radio networks. Various transmission standards have been established for these networks, such as e.g. Global System for Mobile communication (GSM), Bluetooth, WiFi or Near Field Communication (NFC). In these cases, the transmission line connected to the field device is a coaxial line of an antenna, via which the signals are sent and/or received wirelessly.
Both for wired as well as also for wireless information transfer, an adapter is required, via which the electronics arranged within the housing of the field device can be connected to a transmission line located outside of the housing. The adapter must have at least one means, by which the conductors of the transmission line are led through an opening in a housing wall of the housing of the field device.
In such case, field devices usable in explosion-endangered regions must meet special safety requirements. These have the goal of preventing spark formation in the field device, or preventing that sparks occurring in the interior of the field device affect the environment. This goal is attainable in different ways, which are referred to in corresponding European standards as ignition protection classes. A protection class entitled ‘pressure-resistant encapsulation’ (Ex-d) provides that devices must have a pressure-resistant housing, to assure that a spark in the interior of the housing, namely a spark possibly even triggering an explosion in the interior of the field device, cannot ignite an explosive medium located outside of the field device.
In order to achieve a pressure resistant encapsulation, cable feedthroughs extending through the housing walls of these devices must be correspondingly embodied. For this, today, e.g. pressure resistant glass- or ceramic-sealed cable feedthroughs are applied. These usually comprise a metal support, which has a traversing bore. In the bore is a filling of, glass or ceramic, wherein at least one conductor extends through the bore. In the case of these cable feedthroughs, the filling is directly exposed to the pressure of an explosion in the interior of the housing, Correspondingly, the connection between the outer lateral surface of the filling and the inner wall of the bore must be sufficiently strong to withstand the pressure of the explosion. The manufacture of these cable feedthroughs is, consequently, comparatively complicated and expensive. Moreover, the material combinations of filling, conductor and support are limited to those, between which sufficiently pressure resistant connections can be produced. Moreover, there is in the case of these cables guides the problem that the materials usable as filling are, as a rule, hard and/or brittle materials, which have coefficients of thermal expansion, which differ significantly from the coefficient of thermal expansion of the metal support externally surrounding the filling. Due to the different coefficients of expansion, thermomechanical stresses form in these cable feedthroughs as a function of the ambient temperature. In such case, there is, especially in the case of strong temperature changes or extremely high or low environmental temperatures, the danger that stress cracks can arise in the filling, which degrade the sealing and pressure resistance of these cable feedthroughs.